This invention relates to a device which determines the position of a rotatable element. More specifically, it relates to a relatively small, highly precise, position detector suitable for use in precision instruments. Additionally, the position detector is capable of providing indentical position indicating outputs for different angular positions of a rotatable element when such outputs are required due to symmetry of the rotatable element or for any other reason.
One such machine having a symmetrical rotatable element is an ophthalmic instrument called a refractor which is described in detail in commonly-assigned U.S. patent application Ser. No. 06/513,707, now U.S. No. 4,523,822. In that device, the outputs of an optical position detector are used to provide a digital readout of the angular orientation of a motor-driven lens mount, although such detector is operable for a hand-driven mount assembly too. Such an optical position detector and rotatable lens mount are shown generally in FIG. 1.
Referring to FIG. 1, a motor-driven lens mount 12 is rotatably mounted in a housing 14 by a bearing 16. While the assembly shown in FIG. 1 rotates on ball bearing 16, other lens assemblies are friction mounted for rotation. The optical position detector includes an annular code carrier 18 which is rigidly mounted on and rotatable with the lens mount. A positional code carried by the code carrier 18 is read optically through the utilization of a high intensity light source 20 which is rigidly supported in a mount 22 on one side of the code carrier 18 and extends in a generally radical direction with respect thereto. Source 20 is seen to include a light emitting diode (LED) 20a which is optically coupled in light transferring relationship with a transparent rod 20b. The base of the encoder mount supports a linear array 24 of 8 photodetectors in the form of phototransistors mounted on a ceramic substrate 26. These phototransistors extend in a generally radical direction in complement with respect to the code carrier 18 and are aligned with the transparent rod 20b. A mask 28 which has a narrow elongate slit is rigidly secured to the mount and interposed between the light emitting rod 20b and the code carrier 18 such that the slit is laterally aligned with the rod 20b and the phototransistor array 24. In order to determine the angular orientation of the lens mount 12, the code carrier 18 is provided with an 8-bit selectively opaque-transparent code contained in eight concentric rings. The phototransistor array 24 is mounted such that each phototransistor is directly below and aligned with one of the concentric rings on the code carrier 18. Thus, a phototransistor is active or inactive depending upon whether the portion of the ring thereabove is transparent or opaque. If it is transparent, light can pass through the ring to activate the associated phototransistor. The code on the carrier 18 is arranged such that there are two sequentially arranged 180.degree. increments on each half of the ring. This arrangement is provided because the rotatable lens mount 12 contains a cylindrical lens which has 180.degree. symmetry. Thus, it is desirable to provide an indentical output from the code carrier 18 for positions which are diametrically opposite each other, ie. which are 180.degree. apart. Preferably, the position code must be arranged to provide a position defining transition or change in the opaque-transparent portion of the position code for each one degree angular increment of the code carrier.
It has been found that the outputs from the optical position detector described above could be made relatively unambiguous, ie. no spurious outputs during the transition from one angular position to the next adjacent angular position, if the position code on the code carrier were arranged such that there was a transition from opaque to transparent or vice versa in only one of the concentric rings for adjacent angular positions. This is because multiple transitions between adjacent angular positions were found to cause some of the phototransistors in the array to change state before others and hence cause a spurious output from the array when the code carrier was moved from one angular position to an adjacent angular position. Such a spurious output is highly undersirable in a precision instrument, such as a refractor, where the output of the photodetector is being read by an operator to determine the corrective lenses (power and axis) necessary for a patient. In order to make the positional code of the code carrier have a transition in only one ring for adjacent 1.degree. positions, a Gray code arrangement was used where the transparent regions were assigned a logic "20" value and the opaque regions were assigned a logic "1" value. Additionally, since the lens mount contained cylindrical lenses which have 180.degree. symmetry, the code carrier was formed with two identical code regions each ranging from 0.degree. to 179.degree. and arranged such that diametrically opposite positions of the code carrier produced identical outputs from the positions on the code carrier 18, the only adjacent angular positions which required a transition in more than one concentric ring were 0.degree. and 179.degree.. This occured at two positions of the code ring.
The refractor described above is a complex instrument which contains many parts. As such, space is at a premium and the optical position detector by necessity must be very small. An indication of the size of the device can be seen by the fact that the phototransistors are spaced 0.025 inches apart and have an active area 0.018 inches square. This very close spacing of the phototransistors presented a problem in that light from the source which passed through the transparent region of one ring on the code carrier to activate the phototransistor associated therewith was sometimes received by an adjacent phototransistor and was activated thereby. To prevent light passing through one ring of the code carrier from activating more than one phototransistor, it was found necessary to place the code carrier in very close proximity, i.e. on the order of 0.005 inches, to the phototransistor array. However, because the phototransistor array is mounted on a very thin lead frame and each transistor is connected to a lead with a very thin wire, it was necessary to encapsulate the phototransistors, the wires, and the leads in a block of clear plastic in order to prevent the connecting wires from breaking. It was found that if the amount of plastic above the phototransistor array was less than five-thousandths of an inch to permit required close proximity of the code carrier to the phototransistor array, the block did not have sufficient strength to prevent breaking of the lead wires.
Hence, it is desirable to provide a unitary position detector assembly which prevents light passing through a transparent region of one code carrier ring from activating more than one phototransistor while at the same time providing a strong mounting for the phototransistor array.
Additionally, it is desirable to provide a position detector in which the position code of the code carrier has a transition in only one ring for all adjacent angular positions of the code carrier.
It is further desirable to provide a position detector in which the position code of the code carrier can provide identical outputs for multiple angular positions of the code carrier with a transition in only one ring for all adjacent angular positions.